This invention relates to devices for transferring a cryogenic liquid from a stationary source into a rotating machine such as a dynamoelectric machine having a superconducting winding on its rotor.
Cryogenic liquids such as helium have substantial benefits in cooling electrical conductors such as a superconducting field winding on the rotor of an AC generator but their use presents delicate problems in transferring the liquid from a stationary source into the rotating member and in maintaining thermodynamic and mechanical stability under all conditions, including transient fault conditions, for which the machine is designed to be operable. That is, the need is for transfer devices that are not only effective under some ideal set of conditions but which are also effective for a range of perturbations in the system that can be normally expected to occur sometime in operation.
An approach to a fluid transfer coupling for cryogenically cooled rotors is presented in Laskaris U.S. Pat. No. 3,991,588, Nov. 16, 1976. Here, a stationary supply tube runs axially within the end of a delivery conduit, that rotates with the rotor shaft, in a bayonet type of fit. By the means disclosed in the patent, coolant is delivered from the tube to form an annular liquid region on the surface of the conduit by centrifugal action. Coolant is intended to be prevented from escaping between the tube and the conduit by a seal element affixed to the conduit that extends radially inward a greater distance than the anticipated extent of the annular liquid region. This arrangement is not regarded as desirable or effective where the machine can be normally expected to be subject to transient fault conditions in which a pressure buildup occurs in the rotor causing a large mass of liquid to back up in the supply tube and conduit and to substantially flood the entire volume of those channels.
The foregoing considerations were taken into account and solved by the apparatus disclosed in Eckels et al. U.S. Pat. No. 4,356,700, Nov. 2, 1982, in which the bayonet coupling is designed for operation under flooded conditions with a threaded throwback or wind back seal disposed on the interior surface of the rotating conduit proximate the end of the fixed supply tube so that the rotation of the conduit induces fluid flow continuously in a downstream direction as the fluid is carried by the threads on the surface of the conduit. This is a successful arrangement but requires careful consideration of the design of the threaded member in relation to all the possible coolant supply conditions to insure that the threads are continuously effective to move coolant in the downstream direction without any appreciable amount of coolant escaping upstream which would be adverse to the system. Additionally, another aspect of the transfer system that affects the performance of the coupling is that the normal supply of coolant, through the fixed tube, is a churning mixture of liquid and vapor that induces mechanical pulsations giving rise to a critical mechanical natural frequency, hence subjecting the end of the fixed tube, which cannot be readily supported directly, to vibration that can cause it and its coaxial vacuum seal to be abraded by the rotating conduit, which itself may be subjected to some incidental radial motion.
The foregoing problems are addressed and solved by the present invention which utilizes a transfer device of the same general character as that of the abovementioned Eckels et al. U.S. Pat. No. 4,356,700, which is herein incorporated by reference for the entirety of its disclosure, with the improvement thereto of having an upstream seal and bearing device, such as in the form of an annular washer-shaped element, affixed to the inner wall of the rotatable conduit at the upstream end of the threaded member providing the throwback function. The washer has an inner diameter less than that of the threaded member for preventing movement of liquid upstream therefrom and, in addition, the inner diameter surface of the washer serves as a bearing surface for relative motion of the rotatable conduit and the stationary tube. The arrangement preferably locates the washer element closely proximate the downstream end of the coaxial sleeve that is disposed around the stationary tube and vacuum sealed therewith. The washer element extends radially inward from the rotatable conduit past the outer radial extent of the coaxial sleeve on the stationary tube. This allows the seal and bearing device or washer to safely limit and control mechanical vibration to which the stationary tube and its coaxial sleeve are subjected.
What is achieved is an arrangement in which the dynamoelectric machine winding can be designed for certain permissible transient fault conditions during which pressure rise in the rotor causes liquid to reverse flow in the conduit and in the tube with the upstream seal and bearing device preventing reverse flowing liquid from escaping between the outer surface of the tube and the inner surface of the conduit and also doubling as a bearing element for the relative motion of the fixed and rotating members.